1. Field of the Invention
The present invention relates to a semiconductor memory device.
2. Related Art
Recently, an FBC (Floating Body Cell) memory device is available as a semiconductor memory device that is expected as an alternative memory to a DRAM. The FBC memory device has an N-type MOS transistor having a floating body (hereinafter, also “body region”) on an SOI (Silicon On Insulator) substrate, and stores data “1” or data “0” depending on the number of holes stored in the body region. For example, when the number of holes within the body region is large, the data is “1” and when the number of holes is small, the data is “0”.
An FBC memory cell constituted by an FET (Field Effect Transistor) has the following problem called a charge pumping phenomenon. When the memory cell is an N-type FET, a part of electrons within an inversion layer in an on-state of the memory cell is trapped in an interface state at the interface between a gate oxide film and the body region. Holes stored in the body region are recombined with electrons, and disappear. Normally, when a word line is activated between a certain refresh operation and a next refresh operation, data is not rewritten into an unselected FBC memory cell which is selected by a word line but is not read from or written into. Therefore, when on and off of the unselected memory cell is repeated during reading and writing of data in a selected memory cell, holes that are stored in the body region of the unselected memory cell storing the data “1” gradually decrease. As a result, the data “1” in the unselected memory cell changes to the data “0”.
Accordingly, although the FBC memory is not a destructive read-out cell unlike a DRAM, it cannot be called a complete non-destructive read-out cell either. In other words, it has become apparent that the FBC memory is a quasi non-destructive read-out cell.